The present invention relates to a process for nitrating aromatic hydrocarbons.
Aromatic nitro compounds are key intermediates in organic synthesis and occupy an important position in the chemical industry. For example, they are reduced to amines and further reacted to give isocyanates. Thus, nitrobenzene is a starting material for the preparation of aniline, from which methylenediphenyl diisocyanate (MDI), an important building block for the preparation of polyurethane foams, is obtained. Aromatic nitro compounds are furthermore the basis for the preparation of crop protection agents and drug substances, dyes and explosives.
Aromatic nitro compounds are obtained industrially from the corresponding aromatics by nitration with nitrating acid, a mixture of concentrated nitric acid and sulfuric acid. The process has been known since the 19th century. Depending on the concentration, substitution pattern on the aromatic and reaction conditions, one or more nitro groups are introduced on the aromatic. The disadvantage is the production of nitrating acid which is contaminated with organic compounds and has to be disposed of or regenerated by an expensive procedure. There is therefore a need to find economical alternatives to this process which can be implemented on an industrial scale.
WO 99/42433 discloses a process for nitrating aromatic compounds, in which the aromatic compound is reacted with nitrogen dioxide and oxygen under superatmospheric pressure in the presence of a suspended oxide catalyst in an autoclave. An oxygen partial pressure of from 3 to 8 bar is employed. The low conversions in this process are disadvantageous.